FIG. 1 illustrates a transceiver 100 that transmits and receives signals on a twisted pair (TP) 110. The transceiver 100 may be associated, for example, with a local area network (LAN) or a digital subscriber loop (xDSL). The main sources of crosstalk in such a transceiver 100 are usually near-end crosstalk (NEXT) and echo crosstalk. Each transceiver, such as the transceiver 100, transmits a first signal, V1, and receives a different signal, V2, on the same twisted pair 110. V1 corresponds to the transmitted signal generated by the transceiver 100. V2 corresponds to the received signal generated by a second transceiver 120. Since the transceiver 100 knows the transmitted signal, V1, that it has generated, the transceiver 100 employs a “hybrid component” to subtract the transmitted signal, V1, from the voltage (V1+V2) on the twisted pair (TP) 110, to obtain the voltage corresponding to the received signal V2.
Near-end crosstalk results from transmitting and receiving different signals on different twisted pairs 110 and having a signal on one twisted pair interfering with the signal on another twisted pair. Echo crosstalk, on the other hand, is the result of crosstalk on the same twisted pair 110 and of discontinuous impedances along a given path, for example, at each connector. When the transceiver 100 transmits a signal, V1, each impedance discontinuity along the path causes the transceiver 100 to receive a wave or echo back. Thus, a transceiver typically includes a near end cross-talk and echo canceller 200, discussed further below in conjunction with FIG. 2, to address the near end cross-talk and echo cross-talk and to improve the recovery of the transmitted signal.
Conventional near end cross-talk and echo cancellers, generally referred to as cross-talk cancellers, have assumed that a channel is balanced (i.e., each twisted pair is purely differential). It has been found, however, that common mode noise impacts the balance of the differential signals and that the “balanced” signal assumption does not hold well, especially at high frequencies. Conventional near end cross-talk and echo cancellers, however, have not accounted for common mode noise and have failed to exploit the information contained in the common mode component of the received signal. In particular, conventional cross-talk cancellers do not account for differential-to-common mode and common mode-to-differential conversion transfer functions of the twisted pair. One of the main benefits of accounting for common mode noise is mitigation of noise, such as alien cross-talk and RF interference. Such noise sources are not known a priori, and therefore cannot be cancelled using conventional cross-talk cancellation techniques. A need therefore exists for a cross-talk canceller that compensates for common mode noise.